This invention relates to articulated fluid transferring apparatus, and more particularly to marine loading arms and alarm systems for determining the spatial position of the outer end of such an arm with respect to the arm booms or limbs.
Fluid loading arms constructed of articulated pipe are extensively used in the petroleum industry for transferring oil or other fluids between a jetty, wharf, or other loading station and a marine tanker moored alongside. Such an arm generally comprises an inboard boom or limb supported on a vertical riser pipe by pipe swivel joints to facilitate pivotal movement about horizontal and vertical axes, and an outboard boom or limb connected by a pipe swivel joint to the inboard limb so as to be pivotal relative thereto about a horizonal axis. The outer end of the outboard limb is adapted to be connected to a pipe manifold on the tanker located within the reach of the arm, such as by a remotely-controllable coupler device.
When an installation of this type is being designed, minimum requirements are set for the reach of the arm. These requirements are expressed in terms of the maximum horizontal displacement of the tanker parallel to and away from the jetty relative to a datum position, the maximum displacement away from the jetty due to variations in the distance between the tanker manifold and the tanker rail, and the maximum vertical displacement due to variations in the water level and the height of the tanker manifold relative to the water level. These displacements define a three-dimensional space that is rectangular in section when viewed in plan or in elevation, either parallel to or perpendicular to the jetty, and this space is known as the arm's "operating envelope". The arm must be able to accommodate all of these displacements so that a safe and secure connection to the tanker's manifold can be established and maintained within the limits of this envelope.
Most articulated arms are counterbalanced so that when empty they are substantially self-supporting. However, the weight of the oil or other fluid in the arm during use is not counterbalanced, and thus must be supported in part by the tanker manifold to which the arm is connected. Clearly, the stress on the manifold increases with the extension of the arm. In addition, the manifold always faces towards the tanker rail, and the stress to which the manifold can be subjected in a direction perpendicular to the rail, and hence to the jetty, is greater than the stress to which it can be subjected parallel to the rail. The stress parallel to the rail increases with an increase in the slew angle, that is the angle between the vertical plane in which the arm resides and the vertical plane through the riser and normal to the edge of the jetty. Thus, to prevent the stresses on the manifold from exceeding safe limits, the extension of the arm and the slew angle must be limited.
To achieve this limitation, alarm systems have been provided for actuation in the event of the angle between the inboard and outboard limbs exceeding a predetermined limit, or in the event of the slew angle exceeding a predetermined limit. These independent limits result in operating characteristics which are not entirely satisfactory, for they in effect define a space within which the arm can operate that is bounded either by arcuate surfaces or by planes passing through the vertical pivot axis of the arm on the riser. Thus, if a specified rectangular operating envelope is to be accommodated, fairly extensive areas outside this envelope will also be within the operating range of the arm, and the stresses which occur when the end of the arm is in these outside areas can substantially exceed those occurring within the envelope.